1. Field of the Invention
The present invention relates to methods and apparatus for displaying images, and more particularly, to methods and apparatus for increasing the perceived quality of displayed images.
2. Background of the Invention
Color display devices have become the principal display devices of choice for most computer users. The display of color on a monitor is normally achieved by operating the display device to emit light, e.g., a combination of red, green, and blue light, which results in one or more colors being perceived by a human viewer.
In cathode ray tube (CRT) display devices, the different colors of light are generated via the use of phosphor coatings which may be applied as dots in a sequence on the screen of the CRT. A different phosphor coating is normally used to generate each of the three colors, red, green, and blue resulting in repeating sequences of phosphor dots which, when excited by a beam of electrons will generate the colors red, green and blue. As CRT's age, the light output intensity tends to decrease for a given input intensity. Thus, older computer CRT display devices tend to be more difficult to read than new CRTs.
The term pixel is commonly used to refer to one spot in, e.g., a rectangular grid of thousands of such spots. The spots are individually used by a computer to form an image on the display device. For a color CRT, where a single triad of red, green and blue phosphor dots cannot be addressed, the smallest possible pixel size will depend on the focus, alignment and bandwidth of the electron guns used to excite the phosphors. The light emitted from one or more triads of red, green and blue phosphor dots, in various arrangements known for CRT displays, tend to blend together giving, at a distance, the appearance of a single colored light source.
In color displays, the intensity of the light emitted corresponding to the additive primary colors, red, green and blue, can be varied to get the appearance of almost any desired color pixel. Adding no color, i.e., emitting no light, produces a black pixel. Adding 100 percent of all three colors results in white.
Liquid crystal displays (LCDs), or other flat panel display devices are commonly used in portable computer devices in the place of CRTs. This is because flat panel displays tend to be small and lightweight in comparison to CRT displays. In addition, flat panel displays tend to consume less power than comparable sized CRT displays making them better suited for battery powered applications than CRT displays. As the quality of flat panel color displays continues to increase and their cost decreases, flat panel displays are beginning to replace CRT displays in desktop applications. Accordingly, flat panel displays, and LCDs in particular, are becoming ever more common.
Color LCD displays are exemplary of display devices which utilize multiple distinctly addressable elements, referred to herein as pixel sub-elements or pixel sub-components, to represent each pixel of an image being displayed. In displays commonly used for computer applications, each pixel on a color LCD display usually comprises three non-square elements, i.e., red, green and blue (RGB) pixel sub-components. Thus, in such systems, a set of RGB pixel sub-components together make up a single pixel. LCD displays of this type comprise a series of RGB pixel sub-components which are commonly arranged to form stripes along the display. The RGB stripes normally run the entire length of the display in one direction. The resulting RGB stripes are sometimes referred to as “RGB striping”. Common LCD monitors used for computer applications, which are wider than they are tall, tend to have RGB stripes running in the vertical direction. While RGB striping is common, R, G, B pixel sub-components of different rows may be staggered or off set so that they do not form stripes. Other LCD pixel sub-component display patterns are also possible. For example, in some devices, each color pixel includes four square pixel sub-components.
While LCD devices offer some advantages over CRT display devices in terms of power consumption and weight, the perceived quality of LCD displays can be significantly impacted by viewing angle. The amount and color of light incident on an LCD device can also have a major impact on the perceived quality of images displayed on the LCD device. In addition, LCDs tend to have response characteristics which can vary widely from display to display and even among the same model displays made by the same manufacturer.
Perceived image quality of a display device depends not only on the physical characteristics of the display device, but also on the viewer's ability to perceive such characteristics. For example, a viewer with a greater sensitivity to one color will perceive an image differently than a viewer with a greater sensitivity to another color. Viewers without the ability to perceive color at all are likely to perceive the quality of a color image somewhat differently than a viewer who can appreciate the supplied color information.
In view of the above, it is clear that physical display device characteristics, viewing conditions, e.g., ambient light, and a user's ability to perceive various image characteristics all have an effect on the perceived quality of an image. Given the quality of current display devices, it is apparent that for many applications improvements in image quality are not only desirable but also important to insure a user's accurate interpretation of displayed images.
In view of the above, it is apparent that there is a need for new and improved methods and apparatus for displaying images such as text and graphics. It is desirable that at least some of the new methods and apparatus be capable of being used with both CRT as well as LCD display devices. Furthermore, in order to enhance the quality of images for specific viewers it is desirable that at least some display methods take into consideration how specific users perceive the images being displayed to them.